1. Field of the Invention
The present invention relates to tire pressure alarm systems for vehicles equipped with inflated tires.
2. Description of the Prior Art
Many tire pressure alarm systems have been devised which electronically signal a driver when tire pressure falls below a predetermined minimum level. Conventional systems involve small transmitters which respond to a low pressure condition within a vehicle tire by emitting a radio signal. The signal is transmitted to a receiver mounted on the vehicle frame. The receiver detects the existence of the radio signal, demodulates it, and generates an alarm signal which warns the vehicle operator that an inordinately low pressure exists in one or more of the tires of the vehicle. Such a condition is otherwise frequently undetectable by the operator of a heavy vehicle in which tires are mounted in tandem on an axle on each side of the vehicle. This is especially true when the inside tire is flat, since visual observation of such a condition is very difficult. Nevertheless when low pressure conditions such as this exist, the remaining vehicle tires and the vehicle suspension can be severely damaged by continued operation of the vehicle with a deflated tire.
While many different tire pressure alarm systems have been devised, all conventional systems have failed to solve certain perplexing problems, or to operate satisfactorily under certain conditions. In particular, conventional tire pressure alarm systems are typically powered by a direct lead from the vehicle battery. Accordingly, a vehicle in storage, or a vehicle which remains parked for a considerable period of time may experience a loss of pressure in one or more of the tires due to very slow leaks. Such slow leaks typically cause no problems since tires are checked and reinflated with normal vehicle maintenance in a vehicle that is used on a regular basis. However, when a low tire pressure condition exists in a vehicle parked without attention for lengthy periods, conventional tire pressure alarm systems operate just as if the vehicle were in use. Typically, under such conditions, the alarm is to no avail, since there is no driver, and frequently no other interested person in the immediate vicinity of the alarm. As a consequence, the alarm continues to operate and eventually runs down the vehicle battery or alarm transmitter power source.
Another problem with conventional vehicle tire pressure alarm systems is that any radio signals generated in response to a low pressure condition usually are inordinately powerful, and interfere with radio communications in the surrounding area. The Federal Communications Commission rules require all transmitters above 100 milliwatts in power input to be licensed for operation. As a consequence, tire pressure alarm systems requiring such licensing become commercially unattractive due to the governmental regulation and the attention and effort involved in compliance.
A further disadvantage of conventional tire pressure alarm systems is that the transmitters utilized employ crystal oscillators to generate radio frequency signals. While functionally quite satisfactory, such crystal oscillators are fragile, especially when subjected to the shocks and impacts that frequently occur in the operation of heavy trucks and semi-trailers. For this reason there is a high incidence of transmitter failure in conventional tire pressure alarm systems.